Adhesives and adhesive tapes



June 10, 1958 w. E. soHl. 2,838,421

v ADHEsIvEs AND ADHESIVE TAPES Filed Nov. 28. 1956 ADHEsIvEs ANDADHESIVE TAPES William E. Sohl, Birchwood, Minn., assigner to MinnesotaMining and Manufacturing Company, St. Paul, Minn., a corporation ofDelaware Application November V28, 1956, SerialNo. 624,723

11 Claims. (Cl. 11T-22) This application is a continuation-impart of mycopending application Serial No. 5133573, filed June 6, 1955 (now4abandoned).

This. invention relates toga novel adhesive tape characterized"` byhaving a novel'water-soluble-adhesive coating that` isL aggressively andnormally tacky and-hence does not require activation by water, solventsor heat at the time of use. The adhesive adheres' tenaciously with astrong holding force when the tape is pressed against objects having ahydrophilic surface, such as paper, cloth, glass, metal and wood,without the need of more than finger or hand pressure'. The inventionincludes the novel adhesive, per se, as well as novel adhesive tapesembodying it.

The adhesive is water soluble. It is insoluble in and is notswelled bygasoline, oils and hydrocarbon solvents. It adheres to damp or moistsurfaces but not to oily surfaces. It appears to be only slightly tackywhen tested by touching with the ngers since these normally have a filmof oilon the surface of the skin. However, it is `properly termed anormally tacky type of adhesive since it sticks upon contact with avariety ofY common materials, asv noted` above.

The presenttype of adhesive tape is fundamentally different in kindfrom-the Well-known pressure-sensitive adhesivetapes, which have awater-insoluble rubber-resin typeof-adhesive coating that is permanentlyand aggressively tacky. The latter type of adhesive is strongly tackywhen touched with the ngers. It is water-insoluble and hydrophobic anddoes not adhere to damp or moist surfaces. The ordinary types aresoluble in and swelled by gasoline, oils and hydrocarbon solvents, andin fact are commonly coated in solution in a hydrocarbon solvent, suchasl heptane.

Despite the fact that it is soluble in water, the adhesive coating ofthe present tape retains its cohesive and tacky state when exposed'either to dry or humid atmospheric conditions. It does not dry out andlose tack even in an anhydrous atmosphere, nor does it become soft andpasty even in a highly humid atmosphere. Adhesives compounded of glueand glycerine, and the like, have long been known which retain a tackystate when exposed to air having a narrow intermediate humidity range,but it isimpossible to compound an adhesive of that type which is bothtacky and rm over a wide humidity range, or at very low or highhumidities.

Ther present adhesive retains adequate tackiness and internal (cohesive)strength even, when exposed for,- a week or more at humidities, inthe'rangeof O to 90% relative humidity. K

Thisis avery valuablefeatureinasmuch as the adhesive tapes may besubjectedduring manufacture, shipment?, storage and` use-toghumidityconditions, which vary from` very. low; to very. high indifferentloealities; and seasons. 'lheproblemlis not-,fullysolvedbyemploying sealed containers sincethe adhesivetape upon removalis generally exposed to the atmosphere for an appreciable United StatesPatent() period and sensitivity to variations in atmospheric corrditionsis undesirable. l

So far as I have been able to determine, no previously known adhesivetape has had the combination of properties mentioned above incharacterizing my product, and'I believe that it is broadly unique.

Illustrative adhesive tapes of this invention are shown in theaccompanying drawingwherein:

Fig. l is a diagram ofa roll of normally tackyadhesive tape 10 comprisedof a backing lll'coated with-a watersoluble normallytacky dry adhesivecoating 12 formulated as herein-after described.

Fig. 2 is an edgewise diagram of a double-coated splicing tape 20comprised of a porous tissue paper that is impregnated and coated onboth sides withv a normally tacky water-soluble dry ladhesiveformulatedy in accord with this invention, and which is provided on oneside with a removable liner 21 to permit of unwinding from rollsandfhandling duringV application, as in splicing paper in paper-millsand printing plants.

The adhesive tape ofthek type shown in Fig. 1` may have anysuitablevbacking, such as cellophane, metal foil, paper or cloth. Whenthe tape is to be wound directly in rollform without the use of a liner,the back surface of the backing should have ak coating to Which theadhesive does not aggressively adhere, such as a polyethylene coating.Other examples o f suitable coatings are the hydrophobic low-adhesionpolymers described in U. S. Patents Nos. 2,532,011 (Nov. 28, 1950) and-2,607,711 (Aug. 19, 195.2). The strength of hlm-backed tapes can beincreased by embedding a rope-fibre tissue paper, or cloth, or non-wovenrayon or glass filaments in the adhesive layer. Ay high-strengthbundling tape canbe provided, for example, inl which a layeroflinearlyaligned rayon-iilamentyarns is embeddedin the adhesive layer soas to produce a tape having a lengthwise tensile strength of about lbs.per inch width; a suitable backing being a cellophane lm having ahydrophobic backsizing of the type described in the aforesaid patents.Such tape has been found useful as a lumber bundling tape, beingemployed, for instance, in bundlinglumber prior to, treatment with awood preservative solution. These solutions contain solvents` thatdissolve or soften the conventional water-insoluble rubber-baseadhesives of regular pressure-sensitive adhesive tapes, and whenthe tapeis subsequently removed from the lumber a deposit of water-insolubleadhesive isv left on the woodwhich is ditlcult toeliminate. The presenttype of tape utilizes an adhesive which is not attacked by the solvent,and if any residue of adhesive is left on the Wood it canrbe readilyremoved if desired by wiping with a damp rag.

A particularly important embodiment of the present invention is inproviding double-coating splicingtapes to be used for splicing webs ofpaper in printing plants and paper` mills, By double-coated it is meantthat both faces ofthe tape carry the adhesive, so that the tape can beapplied to the butt end of one web of paper and then the` tip portionofthe otherweb of paper can be overlapped to provide a Splice. Thepresent tape in-V stantly bonds tol the paper webs and provides animmediate dry union. Thisl is a valuable feature owing to thedesirability of makngyhigh-speed splices.

The double-coated adhesive tape (as shown in Fig. 2,) can be fabricatedby saturating and coating a shortf fibred porous tissue'paper, whichserves asV the internal support for the adhesive coatings. A featureof'the present water-soluble adhesive composition is that it canr beappliedas/ asolution in a volatile'organic solvent, suchas methanol, soas to avoid subjecting the tissuevpaper-y to Water. Evaporation of thesolvent leaves the tissue impregnated and double-coated with the dry buttacky adhesive. The adhesive webs can be slit into tapes of desiredwidth, which are wound into rolls with a suitable liner, such as a lm ofpolyethylene or a paper coated on both faces with polyethylene, toprevent bonding together of adjacent convolutions of the adhesive tape.

Anoutstanding virtue of this novel splicing tape is that it iswater-disintegratable and the adhesive is soluble in water. Largeprinting plants accumulate a great -amount of spliced paper that hasgone through the presses and has then been cut out so as not to beincluded in the printed matter sent to the customer. Some of the splicesare made at the paper mill, and some are made in the printing plant whenchanging rolls of paper. This spliced paper has a much higher salvagevalue if it can be repulped in a paper mill and included in the furnishernployed in making new paper. The present splicing tape permits of thissince the tissue paper base readily distintegrates and redisperses inthe paper-making process, and the adhesive dissolves in the water and isof a kind and in such minute proportion that it has no adverse elect. Incontrast, water-insoluble rubber-base pressure-sensitive adhesives, ifused, not only seal the tissue paper bers so that they cannot easily beredispersed, but the adhesive does not dissolve or completely disperseand appears in the paper product as small specks of foreign matter.Moreover, these rubber-resin or rubbery base adhesive particles cannotbe eliminated from waste paper even by subjecting it to the drasticcaustic soda and chlorination processes employed for deinking wastepaper salvage.

My novel water-soluble adhesive composition consists essentially of ahomogeneous blend of (A) a solid watersoluble polyvinyl carboxylic acidand (B) a compatible hydroxy-polyalkylene permanent elasticizer, inrelative proportions such that the adhesive in dry form is aggressivelytacky and has elastic cohesive strength.

This adhesive mixture retains the components in their water-solublestate. It is to be distinguished from reaction products resulting fromchemical combination of such components to form water-insoluble alkydresin esters, which products are chemically and physically dfferent frommy product and incapable of the same usage.

A dry layer of the adhesive aggressively adheres to paper when pressedthereagainst, and exhibits aggressive tackiness toward paper and othermaterials having a hydrophilic surface. The dry adhesive has highinternal strength or cohesiveness and is elastic or rubbery in nature,and hence is described as having elastic cohesive strength. It isflexible and it can be peeled from a smooth surface, such as a glasssurface, without splitting, owing to its elastic cohesive nature; incontrast to adhesives that are soft or gooey, or are hard or brittle.

An adhesive solution for coating can be readily prepared by dissolvingthe.abovementioned components in water or in a volatile hydroxylatedwater-soluble organic solvent, such as methanol or similar alcohol,serving as a volatile solvent vehicle and being in proportion to providea suitable coating viscosity. The ingredients can be mixed together atroom temperature to provide the desired adhesive solution.

A preferred example of the polyvinyl carboxylic acid component (A), ispolyacrylic acid. This is a high-molecular weight polymer and isobtained as a white powder resembling starch. Another example is a 50/50copolymer of polyvinyl methyl ether and maleic anhydride, having anaverage of two carboxylic acid groups COOH) after hydrolysis in eachcopolymer unit of the polymer chain structure. Still another example isthe half-ester of this copolymer, in which substantially half of thecarboxylic acid groups have been esteried by reaction with methyl orethyl alcohol to provide methyl or ethyl ester groups.

Preferred examples of the hydroxy-polyalkylene elasticizer component (B)are poly-ethylene glycol and polypropylene glycol of about 1,000 andabout 400 molecular weight, respectively. A further example of suitablehy- .4, droxy-polyalkylene compounds is 2-ethyl-l,3hexanediol having theformula:

HO-CHgCH (CH2CH3 CH OH) CHzCHgCHa these high molecular weighthydroxylated compounds are water-soluble and are fully compatible withthe aforesaid polyvinyl carboxylic acid compounds, and function aselasticizers to produce exible elastic type blends of high internalstrength, in contrast to those plasticizers which merely producesoftening and plasticity. They are permanent type elasticizers, by whichit is meant that they are substantially non-volatile and have a lowdegree of hygroscopicity. The vapor pressure at 20 C. is less than 0.01mm. Hg. Continued exposure for 3 Clays of a thin layer to an atmosphereof relative humidity results in the absorption of no more than 10% byweight of water. These factors are required in order that the dryadhesive stably retain its desired characteristics for substantialperiods. A volatile compound would evaporate from the adhesive and causeloss of desired properties. A hygroscopic compound would cause undueabsorption of water when the adhesive is exposed to humid atmospheres,thereby rendering it too soft, whereas in a dry atmosphere there wouldbe a loss of water that would cause a marked change in properties ofsuch a composition and a loss of tack. Glycerine is too hygroscopic andis unsuitable.

The preferred ratio of poly-ethylene glycol of 1000 molecular weight isparts by weight per 100 parts of polyacrylic acid, but substantiallylower and higher ratios can be used. The preferred ratio ofpoly-propylene glycol of 425 molecular weight is parts per 100 parts ofpolyacrylic acid. An optimum ratio for each combination of componentscan readily be determined by trial tests on samples, and no simple ruleof thumb can be given. For instance, a suitable ratio when 2-ethyl-l,3hexanediol is used as the elasticizer is about 200 parts per 100` partsof polyacrylic acid, or per 100 parts of a 50/50 copolymer ofpolyvinylmethyl ether and maleic anhydride.

Increased internal strength at elevated temperatures can be obtained byincluding in the adhesive mixture a small proportion of a cross-linkingcompound that serves to cure the compounds by forming linkages betweenmolecules and thereby stiffening the molecular gel structure and raisingthe softening temperature.

A preferred type thereof is provided by polyfunctional epoxy compounds,such as are obtained by reacting bisphenol A and epichlorohydrin. Themolecule includes epoxide groups which can react with carboxylic acidgroups present in the polyvinyl carboxylic acid polymer molecules of theadhesive, and thereby provide linkages between the latter molecules. Apreferred ratio is about 0.5 to 1.5 parts by weight of epoxy compoundper 100 parts of polyacrylic acid. The cross-linking reaction shouldoccur in the dry adhesive, and the epoxy compound should be added to theadhesive solution shortly before coating application. The adhesivecoated tape material can be heated for 24 hours at 120 F. to cure theadhesive; however, curing at warm room temperature (80 F.) will bespontaneously completed in one month, and hence can be effected bymerely storing the adhesive tape for that period in a warm room.

Splicing tapes having an adhesive cured by means of an epoxy compound(or equivalent cross-linking agent) are very desirable for use insplicing paper webs that are subjected to elevated temperatures, as inhigh-speed rotary printing presses equipped with ovens for rapidheat-set of printing inks. A sutlicient internal cohesiveness isretained to assure against pulling Iapart or slippage of the spliceunder tension while passing through the oven.

'I'he cured type of adhesive is rendered more resistant to water, but itwill dissolve in water when mechanically agitated or mixed, as occurswhen a splicing tape is subjected to repulping in a paper mill.

- assegni Example l' A` preferred adhesivecoating compositioni'sprepared by mixing together the following:ingredients:

Parts, by weight Polyacrylic acid 100 Poly-ethylene glycol (mol. wt.of1000) 140 Methanol 300 This solution has a solids content ofk about,45% and has a suitable viscosity for readily, saturatng andV coatingtissue paper. Splicing tape was made `by passing a'web of tissue paperthrough a bath of theV adhesive solution and then through an orifice toprovide a coating weight of 810 grains per square yard (dry solidsbasis). The wet web Was laminated to a thin polyethylene film and dried.The composite web was slit and wound into rolls of tape, thepolyethylene liner being on the outside.

In applying the tape, it is unwound from the roll and a strip ofappropriate length is pressed against fthe surface of the paper. Theliner is peeled off and then the Second paper is pressed `against theexposed adhesive surface, resulting in a iirrn splice between thepapers.

Splicing tape made in this Way has been tested under a variety ofcommercial usages and found satisfactory under conditions not involvingtension of the splice lat elevated temperatures.

Example 2 The following is la modifie-d formula useful for makingsplicing tapes that are resistant to elevated temperatures.

Parts by weight The epoxy compound, a curing agent which serves toimpart increased gel strength to the adhesive and which elevates thesoftening temperature, is added to the solution shortly before it isused; Curing is eiected by heatin-g the adhesive coated tissue paper for24 hours at 120 F. A preferred example is a resorcinol diglycidyl ethermade from rescorcinol and epichlorohydrin in the presence of aqueouscaustic soda, having an epoxy equivalent value of about l50. Anotherexample is a diglycidyl ether of diethylene glycol, prepared in asimilar way from diethylene glycol and epichlorohydrin, having an epoxyequivalent value of about 135. The epoxy equivalent is the weight ingrams which contains i6 grams of oxirane oxygen (this being the oxygencontained in the epoxy rings).

Splicing tape of this type has been employed with complete success orsplicing the paper webs passing through high speed rotary printingpresses equipped with ovens for fast heatsetting of the printing inks.The splice does not slip even at temperatures which scorch the paper.

Example 3 This example illustrates the preparation of a `preferredaqueous solution of the adhesive composition.

An aqueous solution ot polyacrylic acid is prepared in a glass-linedkettle by polymerizing acrylic acid in water solution, employingpotassium persulfate as the catalyst, so yas to obtain a productsolution containing about 17% solids and having a Brookfield viscosityin the range of 5,000 to 100,000 cps. at 80 F. Addition is made ofapproximately 0.272 part by weight of polypropylene glycol (mol. wt. of425) to each part by weight of the polyacrylic :acid solution, withconstant stirring at room temperature. Then 0.0035 part by weight ofhexamethylene diamine (as a 70% solution in water) per part by weight ofthe polyacrylic acid solution, is slowly added with continued mixing toneutralize the sulfuric acid that is present as the result ofdecomposition :of the potassium persulfate catalyst during the aforesaidpolymerization process. A dye (such as Malachite Green) may 6 beincluded, if'desired, so that coatingsmf. the adhesive willbelcoloredand ,therebybe made morezreadily. visible.

'I'liisaqueous adhesive solution contains anunreacted mixture ofpoly-propylene` glycol andl polyacrylic acid in a weight, ratio of,approximately` 160:100. The total adhesive solidsconstitute about 34%lby weight off the solution.

Adhesive coatingsl formed by applyingvthis aqueous solution to basesurfacesetnddrying,A have good internal strength at elevated;temperatures withoutl the need of employingI a cross-linking. agent and,not having` been cured thereby, retain. the., propertyyof; readily.dispersing in water.

The adhesive can be cast by coating the solution on a polyethylene film,or yon a paper coated with polyethylene. or other suitable liner, anddrying. The dried adhesive coating can be transferred to another surfaceby pressing the exposed tacky adhesive surface thereagainst and peelingoff the liner, thereby exposing the tacky adhesive surface that wasinitially in contact with the liner. Double-coated adhesive tape can bemade -in this manner by transferring a dried layer of the adhesive toeach face of a tilm, paper or cloth suppont, one liner being temporarilyretained if desired to permit of widing the tape in roll form. Theaforesaid splicing tapes having a tissue paper support can be made inthis manner, thereby avoiding subjecting the tissue paper to the aqueousadhesive solution.

I claim:

1. An adhesive tape having a dry layer of normally tacky water-solubleadhesive that aggressively adheres to paper when pressed thereagainstand consists essentially of a homogeneous blend of a. solidwater-soluble polyvinyl carboxylic acid and a compatiblehydroxy-polyalkylene permanent elasticizer in relative proportions suchthat the adhesive is aggressively tacky and has elastic cohesivestrength.

2. An article according to claim 1 wherein the adhesive consistsessentially of parts by weight of polyacrylic acid and about parts ofpoly-propylene glycol.

3. An arti-cle according to claim l wherein the adhesive is cured with asmall proportion of admixed crosslinking agent that increases thecohesive strength of the adhesive at elevated temperatures.

4. An article yaccording to claim l wherein the adhesive consistsessentially of 100 parts by weight of polyacrylic acid and about 160parts of poly-propylene glycol, and is cured with a small proportion ofadmixed epoxy curing agent which imparts improved heat resistance.

5. A splicing tape comprising a tissue paper impregnated and coated onboth faces with a dry, normally tacky, water-soluble adhesive thataggressively adheres to paper when pressed thereagainst and consistsessentially of a homogeneous blend of polyacrylic acid and a compatiblehydroxy-polyalkylene permanent elasticizer in proportions such that theadhesive is aggressively tacky and has elastic cohesive strength.

6. A water-soluble adhesive composition which in dry form is normallytacky and aggressively adheres to paper when pressed thereagainst andconsists essentially of a homogeneous blend of a solid water-solublepolyvinyl carboxylic acid land a compatible hydroxy-polyalkyleneypermanent elasticizer in relative proportions such that the dry adhesiveis aggressively tacky and lhas elastic cohesive strength. V

7. An adhesive composition according to claim 6 which includes a smallproportion of a cross-linking agent that increases the cohesive strengthof the adhesive at elevated temperatures.

8. A methanol solution of the adhesive composition of claim 6.

9. An aqueous solution of the adhesive composition of claim 6.

l0. An adhesive composition according to claim 6 which consistsessentially of a mixture of 100 parts by weight of polyacrylic acid andabout 160 parts of polypropylene glycol.

` 11. An adhesive composition according to claim 6 which `consistsessentially of a mixture of 100 parts by weight of polyacrylic acid andabout 160 parts of polypropylene glycol, and a small proportion of epoxycuring agent which imparts improved heat resistance.

References Cited in the file of this patent UNITED STATES PATENTS 8Minier Oct. l0, 1950 Dahlquist et a1. Nov. 28, 1950 Kunze May 6, 1952Hendricks Aug. 19, 1952 Seymour Aug. 19, 1952 Hanson Aug. 11, 1953 CassJuly 6, 1954 FOREIGN PATENTS Great Britain July l5, 1936 Great BritainJuly 18, 1953 Australia Feb, 8, 1952

1. AN ADHESIVE TAPE HAVING A DRY LAYER OF NORMALLY TACKY WATER-SOLUBLEADHESIVE THAT AGGRESSIVELY ADHERES TO PAPER WHEN PRESSED THEREAGAINSTAND CONSISTS ESSENTIALLY OF A HOMOGENEOUS BLEND OF A SOLID WATER-SOLUBLEPOLYVINYL CARBOXYLIC ACID AND A COMPATIBLE HYDROXY-POLY ALKYLENEPERMANENT ELEASTICIZER IN RELATIVE PROPORTIONS SUCH THAT THE ADHESIVE ISAGGRESSIVELY TACKY AND HAS ELASTIC COHESIVE STRENGTH.